Reduction of a nitro group of compounds is one of the most important reactions in organic synthesis. Various reduction methods are known, including catalytic reduction using hydrogen in the presence of a catalyst; reduction using a hydrazine compound, an olefin compound, such as cyclohexene, or formic acid, in the presence of a catalyst; reduction using an iron carbonyl compound; reduction using a hydrogenated aluminum compound, such as hydrogenated lithium aluminum; reduction using a hydrogenated boron compound, such as hydrogenated boron sodium, or a combination of a hydrogenated boron compound and a metal compound (e.g. nickel chloride, and copper acetate); reduction using zinc or tin in the presence of hydrochloric acid; reduction using an activated iron powder; reduction using a sulfide, and reduction using sodium hydrosulfite (for example, S. R. Sandler, W. Karo, ORGANIC FUNCTIONAL GROUP PREPARATIONS, Second Edition, Volume I, Academic Press, Inc, 1983, pp. 405-411; and L. C. Larock, COMPREHENSIVE ORGANIC TRANSFORMATIONS, A Guide to Functional Group Transformations, VCH Publishers, Inc, 1989, pp. 411-415).
In recent years, influences to environment of chemical production processes have drawn much attention. As such, there is a demand for a clean chemical reaction that is run under mild reaction conditions; that is reduced in wastes, and that needs least-possible amounts of harmful reaction agents.
For the problems stated above, reduction reactions of a nitro group of compounds have problems enumerated below.
(1) Reduction using an iron carbonyl compound: The reaction agent is toxic and expensive.
(2) Reduction using a hydrogenated aluminum compound: The reaction agent is expensive and is unstable against moisture, which can be dangerous.
(3) Reduction using a hydrogenated boron compound: The reaction agent is expensive and is unstable against moisture, which can be dangerous.
(4) Reduction using zinc or tin in the presence of hydrochloric acid: Disposal of an acidic effluent containing metals, is difficult.
(5) Reduction using an activated iron powder: Much iron waste is generated, and disposal of it is difficult.
(6) Reduction using a sulfide: The sulfide has an unacceptable odor, and disposal of effluents containing the sulfide, is difficult.
(7) Reduction using sodium hydrosulfite: Treatment of waste fluids is troublesome, and the reaction is difficult in an organic solvent system.
(8) As to the conditions of these reactions, an excess amount of reaction agent is used to complete the reaction, in almost all cases, which not only increases cost but also causes complicated work, including disposal of wastes in after-treatment following the reaction.
As a method to solve such problems, use of a catalyst is practical. Catalytic reduction using hydrogen, and hydrazine reduction, in the presence of a catalyst, such as palladium, platinum, or nickel, are well known. These methods, however, cannot be applied to almost all cases of using a substrate in which a low-valence sulfur functional group, such as a sulfide, coexists in its molecule. Further, even contamination by a small amount of a low-valence sulfur compound, in the substrate, can make reduction difficult, in many cases.
In the meantime, as a method to reduce a nitro group to an amino group of compounds, in a reduction method using hydrazine, there are known a method using iron oxides together (for example, Nihon Kagakukaishi (the Journal of Japanese Chemical Society), pp. 858-863 (1978)), and a method using ferric chloride and activated carbon together (for example, JP-A-51-125027 (“JP-A” means unexamined published Japanese patent application), JP-A-5-271175, JP-A-6-135905, JP-A-2002-193899, and Synthesis, pp. 834-835 (1978)). However, these methods make no reference to the reduction of a nitro group of a compound having a low-valence sulfur functional group, such as a sulfide, in its molecule.